This paper introduces and exploits a hybrid numerical approach for fully resolved numerical simulations of reactive mixing in T-shaped microreactors and thereby enables a computational analysis of how chemical reactions interact with convective and diffusive transport. The approach exploits the fast redirection of the flow inside the mixing channel, resulting in a flow field with positive axial flow component everywhere after a short entry zone. This allows handling the axial flow direction as a pseudo-time variable, so that the evolution of the concentration profile can be computed consecutively on successive cross sections, following the main axial flow direction. With this approach the finest length scales, given by the Batchelor length scale, can be resolved for such a reactive mixing process inside a T-microreactor at stationary flow conditions. This allows for a detailed analysis of the mixing state as well as important characteristics of the reactive mixing process like yield and selectivity. The concrete numerical simulations yield local diffusion times inside the reactor, reveal the influence of the strength of the secondary flow on the progress of the chemical reaction and show how local selectivities result from the species transport.

We extend and apply a method for the numerical computation of convective and diffusive mixing in liquid systems with very fast irreversible chemical reaction to the case of unequal diffusivities. This approach circumvents the solution of stiff differential equations and, hence, facilitates the direct numerical simulation of reactive flows with quasi-instantaneous reactions. The method is validated by means of a neutralization reaction which is studied in a T-shaped micromixer and compared with existing experimental LIP-data. Because of their large are-to-volume ratio, microreactors are well suited for fast chemical reactions which are seriously affected by the slow diffusive transport in aqueous media. Numerical computations for different reactor dimensions reveal the fact that, in a dimensionless setting, the obtained conversion is independent of the reactor size, if the flow conditions are the same. This corresponds to an increase of space-time-yield proportional to the square of the inverse scale factor. (C) 2009 American Institute of Chemical Engineers AIChE J, 56: 1406-1415, 2010

Reactive mixing in T-shaped microreactors is studied based on numerical simulations. The flow conditions under consideration are laminar and stationary, but with a rather complex secondary flow which promotes mixing. A hybrid numerical approach is developed which allows for full resolution of all relevant scales and thereby enables a computational analysis of how chemical reactions interact with convective and diffusive transport. The approach is extended to include instantaneous reactions. Results of the numerical calculations are in excellent agreement with local data from experimental measurements. Simulations under different operating conditions help to understand the scaling behaviour of reactive mixing in microreactors.

The large area-to-volume ratio of micro-reactors Gives prospect of better yield and selectivity than for conventional designs, since diffusive fluxes of mass and heat in micro-devices scale with the area, while the rate of changes Corresponding to sources and sinks are proportional to the volume. Indeed, theoretical considerations of the scaling behavior support the fact that micro-reactors allow for faster chemical reactions and provide better thermal control; cf., e.g., [1]. For applications in Chemical Reaction Engineering, the mixing of chemical species is of special interest, since it is an essential condition for chemical reactions to occur.Avoiding large pressure drops, laminar flow fields with secondary flows are chosen to increase the contact area between the inflowing educts. In this laminar but complex flow, the mixing state is determined by the interplay of convective and diffusive transport phenomena and, hence, a thorough CFD-simulation of yield and selectivity of such a reacting flow requires the resolution of the finest length scales of both the velocity and the concentration field. In order to reduce the numerical complexity we employ a simplified mathematical model of the relevant convection-diffusion-reaction equations similar to the parabolized Navier-Stokes system.